In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the wetted printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Three-layer plates, for example, are made ready for press use by image-wise exposure to imaging (e.g., infrared or “IR”) radiation that causes ablation of all or part of the central layer, destroying the bonding to the overlying (typically polymer) layer in the exposed areas. One well-established three-layer dry plate design utilizes an oleophilic substrate, a polymeric (e.g., nitrocellulose) central “imaging” layer, and an inherently oleophobic (e.g., silicone or a fluoropolymer) top layer. Retaining heat generated within the imaging layer is essential to successful imaging of the plate at commercially realistic laser fluence levels. This is easily achieved in the case of polymeric base supports, since polymeric materials typically conduct heat poorly (and, hence, thermal losses into and through this layer are minimal). By contrast, thermally conductive supports, such as aluminum or other metals, pose design challenges. Such supports are commonly employed in plates used on large web presses, such as those used by publishers of newspapers, that do not provide clamping mechanisms to retain printing plates against the plate cylinders. Instead, the leading and trailing edges of the plate are each crimped and inserted into a slot on the corresponding cylinder, so the plate is held against the surface of the cylinder by the mechanical flexion of the bent edges. A second environment favoring use of metal substrates involves large-sized plates. The dimensional stability of plastic- or film-based plates tends to decrease with size unless the thickness of the substrate is increased; however, depending on the size of the plate, the amount of thickening necessary to retain acceptable rigidity can render the plate unwieldy, uneconomical or both. By contrast, metal substrates can provide high degrees of structural integrity at relatively modest thicknesses, so dry plates must typically be manufactured on a different coating line.
One well-known expedient for limiting heat dissipation into a metal base support is to add a heat-insulating polymeric layer between the base support and the imaging layer; see, e.g., U.S. Pat. Nos. 6,096,476 and 6,964,841. In such plates, the heat-insulating layer may persist after cleaning and serve as the ink acceptor. This approach complicates plate manufacture and increases cost not only in adding an extra layer to the plate construction, but in limiting the coating lines that may be used. Wet plates, for example, may be manufactured on coating lines set up to apply two successive polymer layers onto an aluminum substrate. Such lines are unsuitable for plates requiring application of an additional layer.